Conventionally, lead storage batteries have been used for starting an engine and for a back-up power source of a vehicle. Among these usages, lead storage batteries for starting an engine function to supply electricity to various electric and electronic devices mounted on vehicles, in addition to a cell motor for starting the engine. After starting the engine, the lead storage battery is charged by an alternator. An output voltage and an output current of the alternator are set so that SOC (state of charge) of the lead storage battery is maintained to be 90 to 100%.
In recent years, a demand for an improvement in fuel-efficiency of the vehicle is increasing, in view of environmental conservation. For such demand, vehicles carrying the stop-and-go-system and the regenerative-braking-system have been considered, for example. In the stop-and-go-system, the engine is stopped while the vehicle is idling, and in the regenerative-braking-system, kinetic energy of the vehicle at the time of deceleration is converted to electrical energy, and the electrical energy is stored.
In a vehicle carrying the stop-and-go-system, the lead storage battery is not charged when the vehicle is stopped in an idle stop mode. The lead storage battery sometimes supplies electrical power to devices mounted on the vehicle while in such a state. Thus, in comparison with conventional lead storage batteries for starting engines, SOC of the lead storage battery inevitably becomes low. In a vehicle carrying the regenerative-braking-system, SOC of the lead storage battery has to be controlled to be lower, to about 50 to 90%, since electrical energy is stored by the lead storage battery at the time of regeneration (deceleration).
In any of these systems, charge and discharge (charge/discharge) are repeated frequently with a lower SOC range than ever. Further, an increase in dark current accompanied with vehicle parts increasingly becoming electrically powered, a discharge of the lead storage battery advances while the vehicle is stopped for a long period of time, thereby leaving a possibility for an over discharge.
Therefore, for the lead storage battery to be used in vehicles carrying these systems, service life properties under a usage mode in which charge/discharge is repeated frequently with a lower SOC range need to be improved.
For deterioration factors of a lead storage battery under such usage mode, an insufficient charge due to a lowering of chargeability of the lead storage battery can be mentioned, mainly. Since a charge system of a vehicle is based on constant voltage control, when chargeability of the negative electrode plate is reduced, a potential of negative electrode decreases at an initial stage of charge and a voltage rapidly rises up to a preset voltage value, thereby decreasing the current sooner. Thus, a sufficient amount of charged electricity of the lead storage battery cannot be secured, thereby rendering the battery to be in an undercharged state.
For suppressing such deterioration, there has been proposed a method in which a lead alloy layer containing Sn and Sb is formed on a surface of a positive electrode grid of a Pb—Ca—Sn alloy, for example (Patent Document 1). The formation of such layer will suppress deterioration of the positive electrode active material and formation of a passivated layer at an interface between the positive electrode active material and the positive electrode grid.
Also, the Sb which exists on a surface of the positive electrode grid partly dissolves in the electrolyte, and deposits on the negative electrode plate. The deposited Sb on the negative electrode active material will raise a charging potential of the negative electrode plate, and a charging voltage will lower down, thereby improving chargeability of the lead storage battery. As a result, deterioration of the lead storage battery due to insufficient charge during charge/discharge cycles will be suppressed.
This method is very effective in the engine starting lead storage battery which is used while SOC is over 90%, and service life properties will drastically improve.
However, when a lead storage battery is to be used in a vehicle equipped with the above stop-and-go-system or the regenerative-braking-system, that is, when a lead storage battery is to be used in a mode in which charge/discharge is repeated under a lower SOC range, there was a problem in that corrosion easily advances in the tab of negative electrode grid, although the chargeability can be secured. When the corrosion advances in the negative-electrode-grid-tab, current collecting efficiency of the negative electrode plate declines due to a decrease in a thickness of the tab, thereby shortening its service life.
The decrease in a thickness of the negative-electrode-grid-tab also weakens strength of the tab, in addition to the decline in the current collecting efficiency. Especially, in batteries mounted on vehicles, since vibrations and impacts are constantly applied to the vehicle while traveling, deformations of the negative-electrode-grid-tab cause the negative electrode plate to become out of position, leaving a possibility for an occurrence of an internal short-circuit by creating contacts between the negative electrode plate and the positive electrode plate.
Conventionally, regarding the corrosion of the negative-electrode-grid-tab, it has been known that by an exposure of a negative electrode strap and the negative-electrode-grid-tab from an electrolyte to oxygen in air, a welded part of the strap and the tab is corroded to cause a disconnection. However, although the negative electrode strap and the negative-electrode-grid-tab are being immersed in the electrolyte, the negative-electrode-grid-tab is easily being corroded when Sb is deposited in a very small amount on a surface of the negative-electrode-grid-tab by dissolution of Sb included in the positive electrode grid and of Sb included in a positive electrode connecting member comprising a positive electrode strap, a positive electrode pole, and a positive electrode connecting body into the electrolyte.
In Patent Document 2, there has been proposed a lead storage battery in which the positive electrode grid, the positive electrode connecting member, the negative-electrode-grid-tab, and the negative electrode connecting member are formed from Pb or a Pb-alloy not including Sb, and a very small amount of Sb is included in one of the negative electrode grid or the negative electrode active material layer to a degree that would not increase an amount of electrolyte loss. Based on such structure, it has been shown that chargeability and a service life for a deep discharge of a battery are improved to a certain extent, by suppressing a dissolution of Sb existing in the positive electrode plate into the electrolyte, and deposition of Sb on the negative-electrode-grid-tab.
However, Patent Document 2 above as well had a problem in that under a usage mode where a charge/discharge is repeated frequently while SOC is in a low range, the negative-electrode-grid-tab is corroded by a dissolution of Sb in the negative electrode active material layer into the electrolyte, and a deposition of the Sb on the negative-electrode-grid-tab.    Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 03-37962    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-346888